As a rule of thumb, casino will not tell you what accounts are detected and found linked, and through what method were they linked. Thus, as I believe some tries to propose here by trying to understand your phone provider's IP situation, it might be better if you try to propose them that the similarities probably due to dynamic IP scenario [suppose it is, indeed, dynamic] made by your provider instead of asking for proof.

On the other side, it might worth mentioning that casino usually use several parameter to detect and be sure of connections between accounts, not just IP address, be it static or dynamic.